The present invention relates to a hard-material-coated member excellent in durability which is the most suitable as, for example,
(1) a mold for plastic working which is used for forging, pressing or the like and is required to have wear resistance, and
(2) a casting member used in contact with a molten metal, such as a mold used for die casting or casting, a core pin, a piston ring used in an injection machine for die casting, or the like.
Molds obtained by using a metal or steel, such as cold-die steel, hot-die steel, high-speed steel, a hard alloy or the like as a base material have been used for plastic working such as forging or pressing. Methods for the above working are classified into cold working in which working is conducted near room temperature and warm or hot working (hereinafter referred to also as warm-hot working) in which a material to be processed is heated at 400° C. or higher. A mold used in either of these working methods is required to have wear resistance on its working plane.
In recent years, a load on the surface of a mold has been increased with an increase in the strength of a material to be processed, an increase in the precision of a product and an increase of the speed of a molding cycle. Therefore, molds having a working plane coated with a hard material such as TiC or TiN by chemical vapor deposition (hereinafter referred to also as a CVD method) have increased in number. However, as characteristics required of a mold, not only wear resistance but also increased precision of the mold itself has come to be required. Therefore, a CVD method employing a coating temperature of 1,000° C. or higher has become unable to satisfy the above-mentioned requirement sufficiently because the mold is remarkably deformed during the coating and a steel-based mold material involves a problem of heat-treatment distortion caused by heat treatment such as quenching and tempering conducted after the coating.
In such circumstances, physical vapor deposition (hereinafter referred to also as a PVD method) has come to be often adopted which permits coating at a temperature lower than a tempering temperature for a mold material. For example, in the case of cold working, there have been proposed a method in which a base material for a mold in a specific composition range is subjected to a nitriding treatment and then a coating layer of TiN is formed by a PVD method (patent document 1) and a method in which the surface of a mold is coated with a nitride, carbide or carbonitride composed mainly of V, by a PVD method (patent document 2). In the case of hot working, there have been proposed a method in which a pretreatment for a PVD method is specified and coating with CrN or TiAlN is conducted after a nitriding treatment (patent documents 3 and 4) and a method in which a CrAlWN coating film containing Al and W is formed (patent document 5).
On the other hand, steels such as hot-die steel, high-speed steel, stainless steel and the like have been used in casting members used for molding by casting of a molten metal. A metal to be molded that is most frequently used for molding by casting is an aluminum alloy. In the case of the above-mentioned steel materials used in casting members such as molds, the steel material in a portion of the casting member in contact with the aluminum alloy is wasted by melting in a melt of the aluminum alloy to increase the iron content of the aluminum alloy melt and deteriorate the quality of a cast product. Moreover, the waste by melting of the molds and the like causes various operational troubles.
As a measure against the above problem, nitriding on the working plane of a casting member has been often conducted because it gives a deep hardened layer and can be conducted at a very low cost. However, since employment of a high-melting aluminum alloy as a material to be molded has become frequent for the purpose of increasing the strength of a cast product, nitriding has the following disadvantages: N in the hardened layer is easily diffused into an aluminum melt at a high temperature, so that the resistance to the waste by melting is deteriorated by the disappearance of the hardened layer, resulting in rapid progress of the waste-by-melting phenomenon.
In order to solve such a problem, employment of a mold having a working plane coated with a ceramic hardly reactive with a molten metal by physical vapor deposition (hereinafter referred to also as a PVD method) has become frequent. There have been proposed, for example, a method in which a base material for a mold is subjected to carburizing or nitriding and then a coating layer of TiC, TiN or the like is formed by a PVD method (patent document 6), and a method in which the surface of a mold is coated with a Ti intermediate layer and then TiAlN (patent document 7). There have also been proposed methods in which CrN is used as a coating layer (patent documents 8 and 9).
Patent document 1: JP-A-58-031066
Patent document 2: JP-A-2002-371352
Patent document 3: JP-A-11-092909
Patent document 4: JP-A-2003-245738
Patent document 5: JP-A-2005-008920
Patent document 6: JP-A-61-033734
Patent document 7: JP-A-07-112266
Patent document 8: JP-A-10-137915
Patent document 9: JP-A-2001-011599
As to a mold for plastic working, a coating layer of TiC or TiN formed by a CVD method has a high hardness and a relatively large thickness of about 10 μm and hence has satisfactory characteristics, but the precision of a mold having such a coating layer is not satisfactory because of, for example, the deformation of the mold caused by a high coating temperature. On the other hand, the coating layer formed by a PVD method, which has been proposed in any of patent documents 1 to 5, gives a satisfactory precision to a mold but is inferior in adhesion to a layer formed by a CVD method. Therefore, when an attempt is made to impart a sufficient durability to the mold by increasing the layer thickness, the coating layer becomes easily peelable. Accordingly, the function of the coating layer cannot sufficiently satisfy the requirements in the use environment of molds for forging or pressing which has become severe year by year.
As to a casting member, an aluminum alloy melt is cast at a relatively high temperature because of a recent increased speed of casting cycles. Therefore, the use environment of a member used in contact with the melt, such as a casting mold, has become more severe. That is, the following troubles have come to be remarkably caused: the thermal expansion of a mold or other members by a molten metal during casting; and the deflection of the mold or the members by shrinkage thereof by application of a mold release agent after the solidification of a material to be processed or by the solidification and shrinkage of the material to be processed. As a result, the coating layers having the compositions proposed in patent documents 6 to 9 are peeled off during use of a member having the coating layer, and a molten metal infiltrates into the member through the peeled portion and reacts with iron in a base material just under the surface treatment layer of the member to form an alloy. When a portion just under the surface coating layer is expanded by the formation of the alloy, the following phenomenon is observed: the treatment layer is peeled off in a moment, so that the waste of the member by melting proceeds rapidly.
In recent years, die cast products of aluminum alloys or magnesium alloys have come to be used as the boxes of hard discs, personal computers, mobile phones and the like, which are required to be reduced in weight. Therefore, such cast products have been reduced in thickness. In order to reduce the thickness of the cast products, the speed of injection of a melt into a mold has to be increased. The above proposed coating layers have insufficient resistance to wear by a melt. An explanation is given below by taking the case of a casting mold. Since the coating layer is worn away by a fluid in a region near the gate of the mold where a melt is introduced into the mold at a high speed, the mold repairing cycle and the mold life are decreased, and the shape of a cast product is unsatisfactory.
The present invention is intended to provide a hard-material-coated member that is excellent in durability because of the excellent adhesion of its coating film in various use environments. Specifically, the present invention is intended to provide a hard-material-coated member which is the most suitable as a mold for plastic working which is used for plastic working of a metal, such as cold or warm-hot forging or pressing and is free from the above problems involved in a mold required to have wear resistance. Furthermore, the present invention is intended to provide a hard-material-coated member which is the most suitable as a casting member free from the above problems which is used with its working plane being in contact with a molten metal at a high temperature, such as a die casting mold or a core pin.